Conventional prior art illumination optical apparatus include, for example, that disclosed in Japanese Laid-Open Patent Application (Kokai) No. H1[1989]-198759, shown in FIG. 21. With reference to FIG. 22, a light beam b1 from a laser light source 400 is incident a first triangular prism 401 serving as an optical delay element. Prism 401 has an incident surface 401S. A portion of light beam b1 incident triangular prism 401 at surface 401S is reflected therefrom at a location L1 without entering the interior thereof. The remaining portion of the light beam enters the interior of prism 401 travels a triangularly shaped delay optical path DOP1, and thereafter returns to location L1.
The portion of light beam b1 that returns to point L1 exits therefrom along the same optical path as the portion of light beam b1 reflected from surface 401S. The remaining portion of the beam within prism 401 once again passes through delay optical path DOP1, and returns to location L1.
In this way, light beam b1 from laser light source 400 is temporally split into a plurality of light beams (theoretically, an infinite number of light beams) at optical delay element 401. Any two temporally consecutive light beams will have an optical path length difference equal to the optical path length of delay optical path DOP1. Furthermore, the optical path length of delay optical path DOP1 is set to be not less than the temporal coherence length of light beam b1 from laser light source 400.
In this manner, a light beam b2 is formed from light beam b1 Light beam b2 is incident a second triangular prism 402 serving as a second optical delay element. Triangular prism 402 has a constitution similar to that of triangular prism 401, the only fundamental difference being that the optical path length of the triangularly shaped delay optical path DOP2 thereof is set to be twice DOP1. Accordingly, a light beam passing through first optical delay element 401 is thereafter temporally split into a plurality of light beams at second optical delay element 402. Any two temporally consecutive light beams exiting from second optical delay element 402 are imparted with an optical path length difference that is twice the optical path length difference of first optical delay element 401.
In this manner, a light beam b3 is formed from light beam b2. The former is incident a fly-eye lens 403, which forms a secondary light source image SL1 comprising a multiplicity of light source images at the rear focus thereof. A light beam b4 from secondary light source image SL1 passes through a condenser lens 404 and illuminates, in superimposed fashion, a mask 405 set at a plane of illumination P1.
As described above, the conventional illumination optical apparatus shown in FIG. 22 permits reduction of coherence even when a coherent light source is employed. This is achieved by creating a series of light beams successively generated using a first optical delay element 401 and a second optical delay element 402 with optical path length differences not less than the coherence length.
Considering the peak width at half height of laser light source 400 to be D1 and wavelength to be .lambda., the temporal coherence length tc is in general given by: EQU tc=.lambda..sup.2 /D1.
If .lambda.=248 nm and D1=0.8 pm, then tc=77 mm; if .lambda.=248 nm and D1=0.6 pm, then tc=103 mm.
In principle, the light within an optical delay element could make an infinite number of passes over the delay optical path before exiting the optical delay element, based on half mirror reflectance, reflecting member reflectance, and so forth. However, half mirror reflectance is typically set at on the order of between 33% and 50%. Setting the reflectance at such a value allows the light to exit the optical delay element after between roughly two or three passes, assuming down to on the order of 1% of the light energy is to be used.
As described above, the value employed for coherence length has conventionally been a function of the entire spectral distribution of the light source. However, upon using an excimer laser and attempting to obtain uniform illumination therefrom, it has been found that the expected uniform illumination cannot be obtained despite the use of coherence reduction means such as described above.
Furthermore, with a conventional illumination optical apparatus such as described above, the delay optical path of the optical delay element is triangular in shape. Thus, the light beam incident the optical delay element can become offset in parallel fashion from the reference optical axis due to the influence of, for example, vibration of the apparatus or the like. In this case, the light beam entering the interior of the optical delay element will no longer return to the location at which it was originally incident. As a result, the optical path of the light beam reflected from the surface of the optical delay element and will no longer be coincident with the optical path of the light beam that enters the interior of the optical delay element, makes just one pass through the delay optical path thereof, and exits therefrom. Consequently, the optical paths of the series of light beams successively generated by way of the two optical delay elements will no longer mutually coincide. Instead, they will move progressively farther away from the reference optical axis. Thus, with the conventional illumination optical apparatus as described above, instabilities arise with respect to vibration. Further, it is difficult to carry out optical adjustments on the apparatus.